US 7758721 B2
A chip pretreatment process which comprises conveying the feed material through a compression screw device having an atmosphere of saturated steam at a pressure above about 5 psig, decompressing and discharging the compressed material from the screw device into a decompression region, feeding the decompressed material from the decompression region into a fiberizing device, such as a low intensity disc refiner, where at least about 30 percent of the fiber bundles and fibers are axially separated, without substantial fibrillation of the fibers. In a more specific form the invention is directed to a process for producing mechanical pulp, including the steps of fiberizing wood chip feed material in a low intensity disc refiner until at least about 30 percent of the fibers are axially separated with less than about 5 percent fibrillation, and subsequently refining the fiberized material in a high intensity disc refiner until at least about 90 percent of the fibers are fibrillated. In another form the invention combines chip fiberizing with chemical treatments, for improving the pulp property versus energy relationships.
1. A process for producing paper making pulp from wood chip feed material, comprising:
(a) conveying said feed material through a compression screw device having an atmosphere of saturated steam at a pressure above about 5 psig;
(b) fully decompressing and discharging the compressed material from the screw device into a decompression region;
(c) feeding the decompressed material from the decompression region into a fiberizing device having an atmosphere of saturated steam, wherein at least about 30 percent of the fiber bundles and fibers are axially separated without substantial fibrillation of the fibers; and
(d) feeding the fiberized material into at least a primary fibrillating device to produce papermaking pulp.
2. The process of
3. The process of
4. The process of
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7. The process of
8. The process of
in step (c) the fiberizing is performed with low intensity in a first disc refiner; and
in step (d), the fibrillating is performed with high intensity in a second disc refiner and optionally fibrillation is continued in a third disc refiner.
9. The process of
in step (c) the fiberizing is performed in a low intensity disc refiner; and
in step (d) the fibrillating is performed in chemi-mechanical process.
10. The process of
11. The process of
12. The process of
13. The process of
14. The process of
15. The process according to
16. A method for producing mechanical pulp from wood chips stored in a bin at atmospheric pressure, comprising:
(a) feeding the chips from the storage bin into a transfer conveyor device having a user-controlled variable conveyance time period during which the chips are exposed to an environment of saturated steam at a pressure above 5 psig;
(b) compressing and then fully decompressing the chips in an environment of saturated steam at a pressure above 5 psig;
(c) defibrating the decompressed chips until at least about 30% of the fibers are axially separated with less than about 5% fibrillation; and
(d) separate from step (c) fibrillating the defibrated fibers in a high intensity refiner to produce mechanical pulp.
17. The process of
18. The process of
This application is a continuation of, and claims priority from, U.S. patent application Ser. No. 10/485,916, now U.S. Pat. No. 7,300,541, filed Feb. 5, 2004 which is the U.S. national phase of International Application PCT/US03/22057, filed Jul. 16, 2003, which claims priority under 35 U.S.C. Sec. 119(e) from U.S. App. No. 60/397,153 filed Jul. 19, 2002, the disclosures of which are incorporated by reference.
The present invention relates to the production of papermaking pulp from wood chip feed material, and particularly to mechanical refining and chemi-mechanical refining.
Efforts have been ongoing for decades to improve mechanical refining techniques (including chemi-mechanical refining) for producing papermaking pulp from wood chip feed material with decreasing specific energy requirements. A significant advance toward this objective was achieved by the present inventor in the mid 1990's, by the development of the “RTS” process, as described in U.S. Pat. No. 5,776,305, granted on Jul. 7, 1998, for “Low-Resident, High-Temperature, High-Speed Chip Refining. This development was directed to the relationship between chip pre-heat environment and high consistency primary refiner conditions, whereby a window of pre-heat residence time, pre-heat saturated steam temperature (pressure) and high disc refining speed produced a noteworthy reduction in specific energy required to achieve commercial strength properties, while retaining satisfactory optical properties.
A significant further development by the present inventor is the “RT Pressafiner” pretreatment, upstream of preheating and primary refining, as described in International Patent Application No. PCT/US98/14710, filed Jul. 16, 1998, for “Method of Pretreating Lignocellulose-Containing Feed Material”. According to the RT Pressafiner development, chip feed material received, for example, from an atmospheric pre-steaming bin, is first conditioned at elevated temperature and pressure for a controlled period of time, and then highly compressed at elevated temperature and pressure, whereupon the pretreated chips may be conveyed directly into the preheater portion of a primary refiner, or retained in an atmospheric bin until subsequent feeding to the preheater of a primary refiner.
The combination of the RT Pressafiner pretreatment with the RTS primary refining, produces an exceptionally energy efficient mechanical refining system, due largely to the significant extent of axial separation of the fibers in the chips fed to the primary refiner. Although the RT Pressafiner pretreatment method and apparatus has been highly effective in producing axially separated fibers (i.e., separated along the grain), there appears to be an upper limit on axial separation of about 25-30 percent of the total chip mass.
It is thus an object of the present invention to provide apparatus and method for producing at least about 30 percent axially separated fibers in the chip feed material during pretreatment upstream of the preheating section of a mechanical refining system.
It is a further object that this high degree of axially separated fibers be achieved while retaining the benefits of the apparatus and method described in International Application PCT/US98/14710, i.e., maceration of chip structure with minimal damage under pressurized inlet conditions, reduction in refiner energy consumption, good extractives removal, improved chip size distribution for refiner stability, and improved impregnation of chemicals, while achieving significant further reduction in required specific energy for producing satisfactory quality papermaking pulp.
This object is achieved in a chip pretreatment process which comprises conveying the feed material through a compression screw device having an atmosphere of saturated steam at a pressure above about 5 psig, decompressing and discharging the compressed material from the screw device into a decompression region, feeding the decompressed material from the decompression region into a fiberizing device, such as a low intensity disc refiner, where at least about 30 percent of the fiber bundles and fibers are axially separated, without substantial fibrillation of the fibers.
In a more specific form the invention is directed to a process for producing mechanical pulp, including the steps of defibrating or fiberizing wood chip feed material in a low intensity disc refiner until at least about 30 percent of the fibers are axially separated with less than about 5 percent fibrillation, and subsequently refining the fibrated material in a high intensity disc refiner until at least about 90 percent of the fibers are fibrillated.
The preferred apparatus for pretreating wood chips according to the invention, includes a pressure housing having an inlet end and a discharge end, a screw press formed in the housing such that the screw press receives material from the housing inlet and advances the material along a rotating screw shaft to compress the material, and a fiberizing device such as a mechanical refiner rotor, optionally within the same housing, which receives material from the screw press and fiberizes the material. Preferably, the screw shaft is axially aligned with the rotor shaft and the screw shaft rotates at a lower speed than the rotor shaft. For example, the screw shaft can rotate at a speed in the range of about 70-100 rpm with the rotor shaft operating at a speed in the range of about 800-1800 rpm.
In an alternative embodiment, the screw shaft and the rotor shaft need not be coaxial, or even in the same horizontal plane. Moreover, the screw and the rotor can be in distinct housings, such that the chips in the decompression region are directed through a chute or the like or conveyed into the inlet of the fiberizing refiner.
Preferably, the single or plural housings are maintained at a saturated steam pressure in the range of about 5-30 psig.
The material discharged from the fiberizing device has, in effect, been “resized” from chips to short, grass-like strands that have been separated along their grain axes into smaller fibrous particles.
It can be appreciated that, although the use of a pressurized pretreatment device, such as a pressurized screw, is known from the RT Pressafiner method, and certainly fibrillating chip material in a primary or secondary refiner is known, a novel and significant aspect of the present invention is the inter-positioning of a highly effective but low energy consuming fiberizing device in the pretreatment process, e.g., in the form of a mechanical refiner, which achieves high fibration without expending the energy required for substantial fibrillation. A premise of the invention is to maximize separation of the fibration and fibrillation steps of the thermomechanical refining process. The latter step is the most energy consuming, and requires efficient energy transfer at high intensity conditions to minimize total energy consumption.
The present invention is highly effective in achieving energy reduction. If one ultimately desires essentially 100 percent fibrillation via conventional mechanical refining, and the feed material is pretreated according to the known, e.g., RT Pressafiner method, the primary mechanical refining must first fiberize the chip material and then initiate fibrillation of the fibers, using design parameters that are especially adapted for the more difficult fibrillation of the fibers. With the present invention, well over 30% of the fibers, and in most instances, at least about 75% of the fibers, are axially separated (fiberized) with, preferably, a low intensity refiner or the like that is highly efficient in fiberizing (but not fibrillating). The fiberized material thus has no measurable freeness. When the fiberized material is then processed by the high intensity refiner, the higher intensity (and thus high energy level) is not wasted on the fiberizing, but rather can all be directed to fibrillating the fibers.
The present invention achieves a much higher level of axial fiber separation as compared with conventional chip presses, even as improved by the RT Pressafiner pretreatment. Fiberizing in a pretreatment fiberizing device permits fiber orientation while the fibers experience the stress-strain cycles necessary to axially separate the fibers. Pressurization permits chip size reduction in the pressing and fiberizing zones with minimal damage to the chip structure. There is a gradual transition from the pressing zone to primary refining, and this achieves axial fiber separation in a controlled manner. Moreover, higher levels of extractive removal can be achieved due to both the pressurized environment and a reduced size distribution. Furthermore, water or chemical liquor impregnation is improved.
Primary refining (fibrillating) in the production subsystem is improved, in that significantly lower specific energy is required for a given freeness, due to the high level of axially separated fibers feeding the primary refiner. This permits the lowest installed energy requirement for a given plant capacity. Moreover, increased primary refiner capacity can result from higher available plate surface area, i.e., the breaker bar zone can be substantially reduced or eliminated because a fiber material rather than chip material is sent to the primary refiner. In addition, the primary refiner load stability is improved due to the reduction in the bulk density of the feed material. The pulp property/specific energy relationships can be adjusted by the level of chip fibration achieved in the pretreatment. Finally, the parameter windows for the RTS primary refining process can be further adjusted to optimize refining for fibrated inlet material rather than merely size reduced or intact wood chips.
In general, the present invention may be alternatively formulated to comprise, consist of, or consist essentially of, any appropriate steps or components herein disclosed. The present invention may additionally, or alternatively, be formulated so as to be devoid, or substantially free, of any steps, components, materials, ingredients or species used in the prior art compositions or that are otherwise not necessary to the achievement of the function and/or objectives of the present invention.
The preferred embodiments will be described below with reference to the accompanying drawings, in which:
The pretreatment subsystem 14 according to the invention, includes a pressurized rotary valve 20, for maintaining pressure separation between the preprocessing subsystem 12 and the balance of the pretreatment subsystem 14, a pressurized compression device 22, such as a screw press, a decompression zone or decompression region 24 which may be part of the screw press or connected to the discharge of the screw press, and a fiberizing device 26, such as a disc or conical refiner.
According to the preferred embodiment of the invention, the environment within the compression device 22, the decompression zone 24, and the fiberizer 26 are all maintained at a saturated steam atmosphere in the range of about 5-30 psig. However, as a minimum, the compression device 22 operates in this environment. Preferably, as shown in
For purposes of the present invention, it should be understood that the chips would experience a volumetric compression in the ratio of about 2:1 to about 4:1 in the compression device 22. This increase in feed material density is then rapidly reversed by decompression in the decompression zone 24 which refers to release of chips at the discharge with a reduction in feed material density approaching the density of the feed material prior to entering the pretreatment subsystem 14.
In the embodiment of
In the embodiment shown in
With reference again to
Preferably, the chip feed material is fed to the compression screw 22 at a consistency in the range of about 30-50%, the decompressed chips are fed to the defibrating device 26 at a consistency in the range of about 30-50%, and the material is fiberized at a consistency in the range of about 30-40%.
It should be appreciated that a variety of disc refiners and conical refiners are well known in the field of both low and high intensity mechanical refining and that the further details regarding the orientation of the opposed refining surfaces, and the pattern of bars, grooves, or the surface irregularities formed thereon, may be selected according to known parameters. However, further development of the present invention with a focus on determining subtle relationships between the fiberizing conditions and the compression screw, or between the fiberizer and the primary refiner, may lead to the discovery of especially effective refiner fiberizing characteristics which are not presently known to the inventor.
The embodiment of
It is further well known that, for a disc refiner, the feed material is conveyed axially to the center of the disc, or “eye” where the material is then redirected radially outward through the space between vertical, or substantially vertical discs. For conical refiners, the material is merely conveyed to the “apex” of the cone, where it can readily follow the oblique path defined by the increasing diameter of the conical section.
Designers of mechanical refining systems can readily implement the various embodiments of the inventive pretreatment subsystem with known technology for the options of one or plural housings, one or plural drive shafts (whether or not connected to each other), one or plural drive motors, and/or one or plural pressures.
The essence of the invention is that the chip material upstream of the primary refiner 82, is defibrated or fiberized without substantial fibrillation. In this context, fiberizing refers to the condition in which fiber bundles (shives) and fibers are axially separated, but not enough energy is transferred to peel off fiber wall material. The removal of fiber wall material is referred to as fibrillation. According to the invention, the early wood and late wood components absorb energy (mostly early wood during the initial stages of refining), and the energy absorbed is sufficient for initiating axial separation of the wood fibers, but insufficient for any appreciable peeling of fiber wall material.
Thus, according to the invention, the chip material is fiberized to the extent that at least 30 percent, typically in the range of about 40-90 percent, of the fiber bundles and fibers are axially separated, with no or very little (i.e., less than about 5 percent) fibrillation.
Such fiberizing without fibrillation is preferably achieved in a low intensity refiner 26, which is commonly understood in the industry as referring to disc rotation speeds of no greater than 1800 rpm for single disc and no greater than 1500 rpm for double disc refiners and about 800 to no greater than 1800 rpm for conical refiners. Qualitatively, intensity is a consequence of the energy imparted to the fiber per impact with a bar structure on the plate in the refining zone. Such energy is typically defined theoretically in units of GJ/t per impact, but a number of other parameters come into play. For present purposes, the above disc refining speeds or a specific energy between about 100-200 kWh/MT will be sufficient indicators of low intensity refining. An extruder screw device may also be suitable for fiberizing chip material without substantial fibrillation.
The degree of fiber separation, and the degree of fibrillation, can be measured by microscopic analysis, such as optical or scanning electron microscopy (SEM) in a manner well known in this field of technology.
Referring again to
As noted previously, immediately before the discharge of the screw press 22, a very high density of wood chip feed material is formed in the restricted annulus and this can form a plug which establishes a barrier between the compression screw 22 and the discharge region 24 which is not only impermeable to fluid flow, but also to steam pressure. For this reason, with a high compression ratio in the screw press 22, a pressure difference can be maintained as between the screw press 22 and the fiberizer refiner 26. For example, 1.0 bar pressure (about 15 psig) can be maintained at the screw inlet 42, and 1.5 bar (about 22 psig) in the fiberizer refiner 26, as well as the condition discussed above, where the screw inlet 42 is maintained in the range of 5-30 psig and the fiberizer refiner 26 operates at atmospheric pressure. This option of operating at different pressures can be utilized as another means of optimizing the wood chip softening conditions during pretreatment.
In this regard, it should be appreciated that the softening of the wood chips at elevated temperature and pressure and associated high compression of the pretreatment subsystem 14 achieves only modest defibration. The main purpose of this portion of the pretreatment is to avoid damage to the fibers while the fibers experience one or both of partial fiberizing (under 25 percent), removal of extractives, and improved receptivity to the introduction of chemicals upstream of the fiberizer refiner 26. As noted above, the essence of the invention is achieving a high degree of fiberizing from about 30 percent to approaching 90 percent, without substantial fibrillation before introduction of the fiberized wood chips into a high intensity primary refiner 82.
It should be understood that the following examples are included for purposes of illustration so that the invention may be more readily understood and are in no way intended to limit the scope of the invention unless otherwise specifically indicated.
The pilot system according to the present invention is represented by RTF-RTS, in which the preprocessing 12 and primary refining 16 were in the same equipment as for the reference RT-RTS runs. The number serving as the suffix to “RTF” indicates the speed of rotation of the fiberizing disc according to the invention. For both the reference runs and the runs according to the invention, the number in parentheses as a suffix to “RTS” indicates the primary refiner disc rotation speed.
The present invention was also found to be exceptionally effective for improving chemi-mechanical refining, e.g., with sulfite or alkaline peroxide addition. In particular, for a given amount of sulfite addition to the overall chemi-mechanical process, implementation of the invention with about half the chemicals introduced in the fiberizer device and about half in the regular primary refiner, gives better results than implementing the invention with all the chemicals introduced in the primary refiner. Good penetration of chemicals into the fiberizered material during the controlled retention time before primary refining improves the reaction of the chemicals with the wood constituents. In this context, not only is the presence of a fiberizing device in the pretreatment stage a significant advance in the state of the art, but furthermore, the benefits are enhanced to an even greater extent with the introduction of chemical reagents in the fiberizing device, especially if there is a delay (retention time) between the fiberizer discharge and the primary refining. Impregnation of chemicals in the fiberized material improves the efficiency compared to impregnating wood chips or macerated chips, due to the higher exposed surface area of the fiberized material for chemical penetration.
A study was conducted on a source of white spruce chips to evaluate the effect of combining extended chip defibration with an acid sulphate chemical treatment. A control RTF-RTS refiner series was initially produced. Two series were then produced with the chemical treatment applied at the fiberizer refiner. The first RTFc-RTS series was produced with the fiberizer refiner pressurized at 1.5 bar and the latter series with the fiberizer refiner at atmospheric conditions. A final TMP series was produced for comparison at conventional refining conditions. The retention time and refining pressure for the TMP series was 3 minutes and 2.8 bar; the chips were destructured using RT-chip pretreatment prior to refining. Table 3 presents the specific energy consumption, tear index and tensile index results.
Addition of the chemical treatment to the fiberizer refiner resulted in an energy reduction of approximately 8% compared to the control series. The chemical treatment did not impact pulp strength properties. An objective of chip fiberization is to improve the impregnation efficiency of chemithermomechanical pulping. Fiberized chips have more surfaces readily exposed for diffusion of chemicals into the wood structure, which can in turn improve the efficiency of wood impregnation.
The RTFc-RTS refiner series produced with the fiberizer refiner at atmospheric conditions, 0 bar, had significantly lower strength properties. This was most likely a consequence of insufficient heating and softening during chip defibration, resulting in fiber breakage and lower long fiber content.
The RT-TMP refiner series had the highest specific energy requirements, approximately 16% higher than the control RTF-RTS series. The RT-TMP series required over 500 kWh/odmt additional energy compared to the RTFc-RTS series produced at a similar freeness and pulp strength.
A study was conducted to evaluate the importance of defibration temperature on red pine chips. Two RTF-RS series were produced at equivalent operating conditions, except defibration temperature. The first series was produced with the fiberizer operating at a pressure of 1.5 bar and the second with the fiberizer at atmospheric conditions. An application of 3.1% sulfite was applied to both series at the fiberizer refiner. Table 4 presents the results for the two refiner series.
The pine pulps produced with the fiberizer at atmospheric conditions had significantly lower long fiber content and strength properties. The red pine was therefore more sensitive to thermal heating during wood defibration than spruce.
The shive content of the material fiberized at 1.5 bar and 0 bar were 49.1% and 64.0%, respectively. Microscopic analysis of the fiberized chips produced at atmospheric conditions revealed considerable fiber breakage.
A study was conducted to evaluate the effect of the chip pretreatment on spruce AP-TMP pulp properties. Two AP-TMP refiner series were produced, with and without RTF-chip pretreatment. The primary refiner disc speed and operating pressure for both series were 2300 rpm and 2.8 bar, respectively. Table 5 presents the alkaline peroxide application levels and pulp property results for the two refiner series.
The pretreated RTF AP-TMP pulps had approximately 2 mNm2/g higher tear index and 10% higher long fiber content. The tensile strength was similar for both series at a given freeness. The control AP-TMP series had 2.5 points higher brightness and lower scattering coefficient, mainly due to a higher application of alkaline peroxide. It is also noted the fiberizer refiner was operated at 1.5 bar. Operation of the fiberizer refiner at lower pressures and even atmospherically is advantageous for maximizing the bleaching response; such conditions are possible without strength degradation if the chips are partially impregnated in the chip press prior to fiberizing.
Results from this investigation show an increase in partially defibrated wood fibers can improve pulp strength properties and the efficiency of refining. The effect is presumed to be mostly a result of separating more latewood fibers, since this component is more easily defibrated in the early stages. The extent of earlywood defibration using the current method was not investigated.
Enhanced separation of the defibration and fibrillation steps appears to be a better approach than combining both mechanisms in a single refining stage. A separation strategy was presented that orients and defibrates fibers gently for maximizing fiber separation without breakage, followed by fibrillation at high-intensity conditions to minimize energy consumption.
A pilot plant analysis was performed to compare the embodiment of the invention with and without sulfite addition on loblolly pine wood chips. The solution used was acid sulfite with a ph of 4.9. The low energy process configuration (RT Fiberizer) consisted of compressing and macerating the wood chips in a pressurized chip press, followed by fiberizing the wood chips in a disc refiner with approximately 120-130 kWh/MT applied. The operating pressure and disc speed of the defibrating refiner was 1.5 bar and 1800 rpm, respectively. The pretreatment process is designated by the prefix RTF. In this study, the effect of the new pretreatment was evaluated in combination with chemical pretreatment.
The fiberized chips were then refined in a pressurized 91 cm diameter single disc primary refiner (36-1CP) operating at RTS conditions. The retention time, pressure and disc speed were approximately 10 seconds, 5.2 bar, and 2300 rpm, respectively. A pressure of 5.2 bar was used instead of 6 bar in the primary refining stage because sulfite was added as a chemical treatment. This reduces the glass transition temperature of lignin, thereby decreasing the necessary refining pressure. The refiner plates used were Durametal 36604 operating in the feeding (expelling) direction to minimize energy consumption. The primary pulps were then secondary refined in the pressurized single disc refiner at a pressure of 2.8 bar and disc speed of 1800 rpm. The refiner plates used in the secondary position were Durametal 36604 operating in the holdback direction. Each secondary refined pulp was tertiary refined in an atmospheric double disc refiner (91 cm diameter) to lower freeness levels. A curve of three or four energy applications was applied in the tertiary refining stage.
The “in refiner” designation refers to sulfite addition only at the refining stages. The “in fiberizer” designation refers to sulfite addition at both the initial defibrating (fiberizer) treatment and mainline (primary) refining.
The series H-runs, in which approximately 2% of the total 3.9% sulfite addition is in the fiberizer, have the lowest energy requirements (see
Comparisons were also made as between the present invention with chemical addition in the fiberizer, versus chemical addition in the refiner following the RT Pressafiner pretreatment according to International Patent Application No. PCT/US98/14710. These series were primary refined to the same freeness.
It can be appreciated that the pretreatment according to the invention had a lower energy consumption to a given freeness. The difference in energy consumption was approximately 200 KWH/MT at freeness of 150 ml. The RTF pretreated series also had a higher tensile index than the RT pretreated series had at a given freeness or specific energy (
The RTF pretreated series also had a higher tear index compared to the RT pretreated series at a given freeness or tensile index (see